This invention relates to hydraulic fuses of the type that pass a system's maximum flow with acceptable pressure loss and which triggers to fully closed position in response to the short interval of liquid decompression flow that accompanies the fused line rupture. To obtain this performance, the lowering of the trigger flow due to the variation of liquid viscosity, or due to external effects such as shock and vibration must not reduce the trigger flow to or below the system maximum flow. The allowable lowering of the trigger flow is dependent on the difference between the mean value of the trigger flow and the system maximum flow. By way of example, if the mean value of the trigger flow is 20 percent above the system maximum flow, the deviation of trigger flow from the mean must be less than 20 percent of the system maximum flow. It is therefore of advantage to utilize a high difference between the mean trigger flow and the system maximum flow and to minimize the effects of viscosity, shock and vibration.
In the related prior art, Jackson U.S. Pat. No. 3,741,241 employs a shutoff device that is biased open by a spring and is driven closed by the pressure drop across the closure device and, Tilman U.S. Pat. No. 3,476,141 and Waterman U.S. Pat. No. 2,821,209 employ a fixed orifice in series with the shut-off device. In both Tilman and Waterman the pressure drop across the fixed orifice produces principally the same force as does the pressure drop across the shut-off device and accordingly, these fuses would be viscosity sensitive even if the fixed orifice were made viscosity insensitive. The viscosity variation encountered in normal liquid service could cause the shut-off flow of these fuses to vary by a factor of greater than 100. In the fuse shown in our copending application Ser. No. 265,596 an upstream, viscosity insensitive orifice is in series with a shut-off device and a second variable but viscosity sensitive orifice. The pressure drop across the second orifice produces a substantially less force than does the pressure drop across the upstream orifice. The effect of the second orifice is further reduced by the large open-state area of the second orifice. The penalty paid for the large area of the second orifice is the required stroke of the moving element. The long stroke contributes significantly to the maximum spring load and thereby limits the bias load that can be used and accordingly, the margin between trigger flow and system maximum flow. The viscosity sensitivity of the second orifice limits the application of the fuse to the lower range of viscosities encountered in current systems.
In the copending application the reduction of area of the orifices with travel causes the fuse to trigger, or to completely close once the shut-off flow is reached. With the fixed series orifice shown in Tilman and Waterman the fuse will close gradually as the flow approaches the shut-off value and trigger-like action does not take place until an uncertain position of the shut-off device is reached. Full triggering gives definition and certainty to the closure point and thereby, improved accuracy of shut-off flow over that provided by Tilman, Waterman or the like.